In recent years, the weights of various components that constitute automobiles are being reduced with the objective of improving the fuel consumption of the automobiles. The method of reducing the weight differs depending on the performance requirements for the respective components. For example, for a framework component, wall thinning is carried out by enhancing the strength of a steel sheet. For a panel component, measures such as substitution of a steel sheet with a light metal sheet such as an Al alloy are taken.
However, a light metal sheet such as an Al alloy is expensive in comparison to a steel sheet. Therefore, utilization of light metal sheets is mainly limited to luxury automobiles. The demand for automobiles is shifting from developed countries to emerging countries, and it is expected that from now there will be demands to achieve both weight reductions and price reductions. Accordingly, for every component, irrespective of the region, there is a demand to achieve increased strength using a steel sheet and a weight reduction by wall thinning.
When wall thinning is exhaustively carried out, it is necessary to meticulously set the sheet thickness and material quality of component parts of each region. However, in this case the number of components increases and the production cost rises. From the viewpoint of enhancing the accuracy of the body shape and improving productivity and the like, it is preferable that the number of components is as small as possible.
Application of tailored blanks is proceeding as a method that, as much as possible, can meticulously set the sheet thickness and material quality of each region and also reduce the number of components.
The term “tailored blank” refers to a press starting material in which a plurality of steel sheets are joined together according to the purpose. Utilizing a tailored blank makes it possible to partially alter the characteristics of a single starting material and to also reduce the number of components. A tailored blank is normally produced by welding together a plurality of steel sheets. Examples of the welding method include laser welding, mash seam welding, plasma welding and high-frequency induction welding.
Tailored blanks produced by welding in this manner are called “tailored weld blanks”. Technology relating to tailored weld blanks is proposed in, for example, Japanese Patent Application Publication No. 7-290182 (Patent Literature 1) and Japanese Patent Application Publication No. 8-174246 (Patent Literature 2).
According to the technology disclosed in Patent Literatures 1 and 2, steel strips of different thicknesses are butted in the width direction and welded by laser welding or the like. However, in a case where tailored weld blanks are produce by applying these technologies, if there is a weld defect at one part of a weld zone, in some cases cracks arise in the weld zone in a pressing process that is after the welding process. In addition, even when a weld zone does not have a weld defect, a hardness difference arises between a weld zone and a base metal portion, and weld undercut portions arise. In such a case, in a subsequent press-forming process, in some cases the stress concentrates at the weld zone during press working, and cracks arise in a portion of the weld zone.
As described above, when welding together steel sheets of different strengths that have different sheet thicknesses by using a welding process that is currently in practical use such as laser welding, mash seam welding, arc welding or high-frequency welding, it is difficult to make the quality of the weld zone uniform, and a weld defect is liable to occur.
Therefore, tailored rolled blanks have been proposed as another kind of tailored blank that does not utilize welding. A tailored rolled blank is a steel sheet of varying thickness on which partial wall thinning has been carried out by rolling. Technology relating to tailored rolled blanks is disclosed in Japanese Patent Application Publication No. 11-192502 (Patent Literature 3), Japanese Patent Application Publication No. 2006-272440 (Patent Literature 4), International Application Publication. No. WO 2008/068352 (Patent Literature 5) and International Application Publication No. WO 2008/104610 (Patent Literature 6).
According to the technology discussed in Patent Literature 3, a steel strip is rolled with work rolls of a special shape to produce a steel strip in which the sheet thickness varies in the width direction. However, when utilizing this technology, it is necessary to prepare a plurality of single-purpose work rolls that correspond to the shape of the steel strip for a tailored blank.
According to technology discussed in Patent Literature 4, a steel sheet of varying thickness is produced without using work rolls of a special shape. Specifically, at least at one location at an intermediate portion in the longitudinal direction of the sheet thickness, rolling is performed by changing the setting of a rolling reduction position so that the sheet thickness changes in a tapered shape within a predetermined length range, to thereby produce a tailored rolled blank. However, in Patent Literature 4, there is no discussion regarding the chemical composition and microstructure and the like of a steel strip to be used for a tailored rolled blank.
In Patent Literatures 5 and 6, a chemical composition of a steel sheet for a tailored rolled blank and a method for producing a steel sheet for a tailored rolled blank are disclosed. According to the technology disclosed in Patent Literatures 5 and 6, using a steel strip having a specific chemical composition, rolling is performed while controlling a roll gap so that the sheet thickness changes in the rolling direction. After rolling, a heat treatment is performed, and the yield strength of a thick-wall portion of the tailored rolled blank is made equal to or greater than the yield strength of a thin-wall portion.
According to the technology disclosed in International Application Publication No. WO 2010/137317 (Patent Literature 7), a steel sheet having a specific chemical composition is subjected to hot rolling under specific conditions to produce a hot-rolled steel sheet. Cold rolling is executed at a reduction of 0.1 to 5.0% on a hot-rolled steel sheet to produce a cold-rolled steel sheet. A heat treatment is executed under specific conditions on the cold-rolled steel sheet to produce a high-strength steel sheet that is excellent in elongation properties.